1. Field of the Invention
The present invention relates to an apparatus, and a related method, for protecting a transistor subject to varying current and voltage, and more specifically to an apparatus and method for protecting a transistor that supplies current and voltage to a variable speed drive motor, such as a drive motor that drives a variable speed centrifuge.
2. Description of the Related Art
Transistor control units, such as those controlling variable speed drive motors, are occasionally subject to overheating, such as during start-up of the motor or other controlled unit. The typical ways of protecting such a transistor control unit generally fall into three main categories. In the first, the temperature of the transistor is directly measured, and control is based upon that measurement. Not only is an additional device required, but also the wiring harness for the controlled device becomes more complex on account of additional wires to the transistor control unit. Moreover, the thermal lag of the temperature sensor and the protected transistor itself jeopardizes the effectiveness of the protection.
The second way to protect the transistor control unit relies on the predictive capabilities of software algorithms; but this approach fails if the software fails on account of a state that was not anticipated. For example, the software may try to start the drive motor when it has stalled under conditions that positively prevent restart. This action may wreck the drive motor or driven centrifuge.
Lastly, the transistor control unit can be integrated into a power regulator of known type with any of several internal monitoring and shutdown features. Unfortunately, the output transistor of such a regulator has a lower saturation voltage than is desired in a transistor control unit for the applications envisioned above.
Moreover, in the particular field of application involving a drive motor and a driven centrifuge, a fairly simple technique involves pulse-width-modulation control of the drive motor by the transistor control unit based on any of several possible feedback control parameters. The implementation of this approach creates electrical and mechanical noise and electromagnetic interference that require expensive, bulky, and otherwise undesirable shielding in many environments. For example, in a surgical environment such as that required for autologous blood salvage by centrifuge, medical personnel and patients object to these characteristics. Nevertheless, the known alternative approach limits starting motor current to a single value that is safe throughout the motor and centrifuge start-up period. Very slow start-up results.
It would be desirable to avoid one or more of these problems.